1. Field of the Invention
The present invention relates to boundary acoustic wave devices using SH (shear horizontal) boundary acoustic waves, and more specifically, the present invention relates to a boundary acoustic wave device having an electrode in the interface between a piezoelectric material and a dielectric material.
2. Description of the Related Art
Various types of surface acoustic wave devices have been used in RF and IF filters for cellular phones, VCO resonators, and VIF filters for televisions. Surface acoustic wave devices use surface acoustic waves, such as Rayleigh waves or first leaky waves propagating along the surface of a medium.
Since surface acoustic waves propagate along the surface of a medium, they are sensitive to the changes of the medium state. Accordingly, in order to protect the surface acoustic wave propagating surface of the medium, the surface acoustic wave element is enclosed in a package having a recess or hole formed in the region opposing the propagating surface. The use of the package having the recess or the hole inevitably increases the cost of the surface acoustic wave device. Also, since the size of the package is larger than the size of the surface acoustic wave element, the size of the surface acoustic wave device must be increased.
In addition to the surface acoustic waves, acoustic waves include boundary acoustic waves propagating along the interface between solids.
“Piezoelectric Acoustic Boundary Waves Propagating Along the Interface Between SiO2 and LiTaO3” IEEE Trans., Sonics and Ultrasonics, VOL. SU-25, No. 6, 1978 IEEE has disclosed a boundary acoustic wave device using such boundary acoustic waves. The structure of the known boundary acoustic wave device will now be described with reference to FIG. 32.
The boundary acoustic wave device 201 has the structure in which an electrode 204 is disposed between a first medium layer 202 and a second medium layer 203. In this instance, an alternating electric field is applied to the electrode 204 to excite boundary acoustic waves to propagate while their energy is concentrated on the interface between the medium layers 202 and 203 and its vicinity. This boundary acoustic wave device, in which an IDT is formed on a 126°-rotated Y-plate X-propagating LiTaO3 substrate, has a SiO2 film with a predetermined thickness arranged over the IDT and the LiTaO3 substrate. In this document, SV+P boundary acoustic waves called Stoneley waves are propagated. Incidentally, this document has disclosed that when the SiO2 film has a thickness of 1.0λ (λ represents the wavelength of the boundary acoustic waves), the electromechanical coupling coefficient is 2%.
Boundary acoustic waves propagate with their energy concentrated on the interface between the solids, and the bottom surface of the LiTaO3 substrate and the upper surface of the SiO2 film hardly have any energy. The characteristics were not therefore varied by the change of the surface state of the substrate or the thin film. Thus, the package having the recess or hole is unnecessary, and the acoustic wave device can be thus downsized accordingly.
“Highly Piezoelectric Boundary Waves propagating in Si/SiO2/LiNbO3 Structure” (26th EM Symposium, May 1997, pp. 53-58 [in Japanese] has disclosed SH boundary waves propagating in a [001]-Si(110)/SiO2/Y-cut X-propagating LiNbO3 structure. This type of SH boundary waves feature an electromechanical coupling coefficient K2 higher than that of the Stoneley waves. In the use of SH boundary waves as well as in the use of Stoneley waves, the package having the recess or hole is not necessary. In addition, since SH boundary waves are of SH-type fluctuation, it can be considered that the strips defining the IDT or reflectors have a higher reflection coefficient in comparison with the case using Stoneley waves. It is therefore expected that the use of SH boundary waves for, for example, a resonator or a resonator filter can facilitate the downsizing of the device and produce sharp characteristics.
A boundary acoustic wave device uses boundary acoustic waves propagating with their energy concentrated on the interface between a first and a second medium layer and its vicinity. In this instance, the ideal thickness of the first and the second medium layer is infinite. However, their thicknesses in practice are finite.
Also, the boundary acoustic wave devices of the above-mentioned documents undesirably produce spurious responses in their resonance characteristics or filter characteristics. Hence, a boundary acoustic wave resonator including such a boundary acoustic wave device is liable to produce a plurality of considerable spurious responses in the frequency region higher than the resonant frequency. Also, a filter made of a plurality of known boundary acoustic wave resonators, for example, a ladder-shaped filter, produces a plurality of spurious responses in the frequency region higher than the pass band, thus degrading the out-of-band attenuation disadvantageously.
This will be further described with reference to FIGS. 33 to 36. An Au electrode was formed to have a thickness of 0.05λ on a 15° Y-cut X-propagating LiNbO3 substrate acting as a first medium layer, and a SiO2 film acting as a second medium layer was deposited to a thickness of 2λ by RF magnetron sputtering at a wafer heating temperature of 200° C. A boundary acoustic wave resonator was thus produced. The electrode 204 included an IDT 204A and reflectors 204B and 204C, as shown in FIG. 33. The impedance-frequency characteristics and the phase-frequency characteristics of the boundary acoustic wave resonator are shown in FIG. 34. As designated by the arrows A1 to A3 in FIG. 34, large spurious responses occurred in the higher region than the antiresonant frequency.
Also, a ladder-shaped circuit shown in FIG. 35 was produced using a plurality of boundary acoustic wave resonators prepared in the same manner as described above, and the frequency characteristics of a ladder-shaped filter thus produced were measured. FIG. 36 shows the results. In FIG. 35, parallel arm resonators P1 and P3 each included an IDT having 50.5 pairs of electrode fingers and an aperture length of 30λ. Series arm resonators S1 and S2 are structured by connecting the same two boundary acoustic wave resonators as used for the parallel arm resonators P1 and P3 in series. Another parallel arm resonator P2 included an IDT having 100.5 pairs of electrode fingers and an aperture length of 30λ. Each λ of the IDTs and reflectors of the parallel arm resonators P1 to P3 was 3.0 μm, and λ of the series arm resonators was set so that the antiresonant frequency of the parallel arm resonator P1 and the resonant frequency of the series arm resonators overlap with each other. The duty ratios of the IDT and the reflectors were each 0.58. The electrode was made of Au and had a thickness of 0.05λ, and the SiO2 film had a thickness of 2.5λ.
FIG. 36 clearly shows that a plurality of large spurious responses indicated by arrows B1 to B3 occur in a region higher than the pass band.